Discharge-pumped excimer lasers are the most important high-power lasers for the ultraviolet spectral range. A conventional discharge-pumped excimer laser includes a laser head having a discharge vessel (laser chamber) which is filled with a suitable gas mixture (laser gas) and contains two elongated electrodes which may have a length of a few ten cm to 1 m, extend parallel to each other and are disposed opposite each other spaced apart a few centimetres. In operation these electrodes are subjected in pulse-like manner to an electrical voltage of a few 10 kV and a rise time of typically 10 to 100 ns in order to generate as homogeneous a discharge as possible between the electrodes. To achieve this, it is also necessary for the discharge space between the electrodes to be irradiated with shortwave ultraviolet light simultaneously, for example by a row of small spark gaps, to ensure an adequate preionization of the discharge path since otherwise instead of the homogeneous discharge only series of individual sparks would form. With appropriate configuration of the electrodes the resulting discharge can obtain a width of a few millimetres up to about several centimeters.
Discharge pumped gas lasers with an off-axis beam path and multiple pass of the laser beam through a discharge area of a gas laser, or through another active laser medium are known (OPTICS COMMUNICATIONS, 15, No. 1, Sep. 1975, 20-25; JP 2-16783 A2). The purpose of this is the provide for angular separation for multiple-pass arrangements and/or better utilisation of the volume of the laser medium, and, thus, the off-axis angle (angle between the beam axis and the longitudinal axis of the active laser medium) is small and/or the subsequent passes of the laser beam are directed through different regions of the laser medium.
The limit to the energy extraction of a short-pulse laser amplifier is set by the cross-section of the amplifier. The maximum energy density is limited to E.sub.sat .times.g.sub.0 /*, due to the combined effect of saturation of the gain and non-saturable absorption (where E.sub.sat is the saturation energy density, g.sub.0 and * are the gain and absorption coefficients, respectively). This sets the theoretical limit of the output energy of the laser amplifier. Up to this limit the increase of the output energy is only possible by increasing the saturation, following the conventional amplifications schemes. However when the energy density already surpasses a value which is dependent on the g.sub.0 /* ratio, further increase of the energy density can only be achieved with significant deterioration of the extraction efficiency and the ratio of the actual gain to small-signal-gain. This is why the energy achievable by the commercially available UV-preionized laser amplifiers was limited to less than 10 mJ (typically 4 mJ and 8 mJ using single and double pass amplification schemes respectively). In the double pass amplification scheme the ASE (amplified spontaneous emission) background increases significantly and spatial filtering is necessary to reduce ASE coupling between the passes.
Using the conventional amplification schemes the beam shape is determined by the cross-sectional distribution and homogeneity of the discharge, resulting in usually poor homogeneity and small beam size in the direction perpendicular to the electric field. A homogeneously pumped wide-aperture amplifier could be a solution to achieve better homogeneity and higher output energy. Unfortunately construction and operation of such lasers are complicated and costly compared to the commercial narrow discharge lasers. On the other hand, discharge-pumped, especially KrF lasers are more efficient when the electric excitation is fast, necessitating small discharge-loop and therefore small discharge cross-section. It is seen, that from the optical point of view an ideal amplifier has large cross-section and small gain, to avoid saturation effects and to operate around the optimum energy density. However these requirements can hardly be fulfilled with efficient discharge-pumped gain modules, having generally a pencil-like active volume.